Analysis of Carbonate Aggregates Using Turner-Fairbank Alkali—Silica Reactivity Tests

The Turner-Fairbank Alkali–Silica Reaction Susceptibility Test (T-FAST) (AASHTO TP 144) and the alkali threshold (AT) Test (ATT) (AASHTO T 416) are novel accelerated chemical tests for determining the alkali—silica reactivity of aggregates and assessing their field performance, respectively. This paper summarizes the background research that led to the refinement of both standard protocols to improve their accuracy and reliability while testing carbonate aggregates. A group of 35 carbonate aggregates of varying mineralogy was selected to validate the proposed modifications and understand the mechanism by which silicon is released in such aggregates during alkali exposure. The results confirmed that the carbonate aggregates’ alkali—silica reactivity character was attributed to the presence of silica particles with an average size range of 2.6 μm to 6.6 μm. A systematic evaluation of the AT values of the sample population revealed that certain aggregates displayed unusually low AT values compared to others within the same reactivity class.